Various clamping devices are known for providing an electrical and mechanical connection from an electrical wire to a metal plate such as a utility box which is separately connected to ground. For example, the utility meter box for an electrical power system, a telephone interface utility box, and a cable television (CATV) box may each be grounded by an elongated ground rod sunk into the ground and connected through heavy gauge wire to the utility box. Typical utility boxes are painted and/or treated with a coating to prevent rusting and to provide a long term life. Drilling a hole through a metal wall of the utility box is highly undesirable in that it may allow foreign matter and moisture into the box, can create a safety problem due to high voltage, can allow corrosion of the metal, and/or may be contrary to local codes.
Numerous clamping devices have been devised to connect a grounded utility box to a separate electrical conductor which is to be grounded so as to eliminate the need for an additional grounding system. Examples of such clamping devices are shown in U.S. Pat. Nos. 4,993,960 and 5,006,074 of the present applicant. Each of these patents show elongated clamps which span a metal utility box which is grounded in order to mechanically and electrically connect the utility box to an electrical wire coupled to other devices which are to be electrically grounded such as a telephone system, a cable TV system or the like. In certain situations, the elongated clamps which span across a utility box are not usable such as when several utility boxes are ganged together or are flush mounted panels. In addition, it is desirable to provide a clamping bracket which is less expensive to manufacture and has utility for a variety of grounding purposes.
Several U-shaped clamping devices, corner brackets and the like are known which are secured to a lip or corner of a utility box in order to couple an electrical wire to the grounded utility box. Generally, such prior art devices have consisted of one or more screws which clamp the device to the utility box, and an additional screw which couples the device to an electrical wire which is to be grounded through the utility box. Typical prior art devices of this type suffer from a variety of problems which have prevented more widespread use. As one or more screws on the device are tightened against a lip or corner of a utility box, continuing tightening can cause the device to "walk" or move across the utility box, causing difficulty in forming an adequate connection. Also, such devices can tilt or skew during tightening of multiple screws.
Because of various code requirements, there should be a separate screw for connecting an electrical wire to the clamping bracket in addition to any clamping screw(s) for connecting the bracket to the utility box. The one or more clamping screws for connecting the bracket to the utility box are tightened first. Then, an electrical wire is connected to the separate screw which is then tightened. This tightening of the separate screw can cause undesirable movement and/or loosening of the clamping screw(s) which have connected the device to the utility box.
To provide grounding protection, the clamping bracket must be able to withstand a fusion test in which high current is passed through the bracket for a predetermined time. Different users as well as standard setting bodies have different requirements. The clamping bracket when mounted to a meter box and serving as a ground connector must survive certain current surges to the extent that a #6AWG solid copper wire connected to the clamp and through which the current is passing will fuse before the integrity of the clamp is compromised. As one example, the bracket should withstand 1,000 amperes for 20 seconds or until the #6 wire fuses (melts and/or breaks open). As another example, 1,530 amperes may be passed for six seconds through the bracket for a total of six times during which the bracket cannot melt, crack or break. A number of prior art brackets are not able to pass such fusion tests and thus are inappropriate for grounding protection against current surges such as caused by lightning.
Other U-shaped clamping devices are known for securing an electrical wire to a metal plate for grounding purposes or the like. For example, U.S. Pat. Nos. 4,828,504 and 4,884,976 of the present applicant each show U-shaped clamps which can connect an electrical wire through a threaded stud having an abrading annular end surface to a metal plate. However, the thickness of the uprights forming the U-shaped clamp prevents the use of such a device when limited space is available. For example, a utility box may include a lip which overhangs a wall and only a narrow space is available for the clamping bracket. Such a narrow space will allow only a thin metal piece to be inserted into the gap to serve as a brace for the bracket.
Because of adverse weather conditions, it is very important for the clamping device to be rugged, as well as capable of forming and maintaining over time a secure mechanical and electrical connection to the utility box. In addition, the clamping device should be inexpensively formed with minimum parts and be capable of simple installation. This combination of features has not been satisfactorily solved in prior clamping devices.